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Scientists have proposed the existence of a new “fifth force” to explain a perplexing challenge to the standard model of cosmology, also known as the Lambda cold dark matter (ΛCDM) model, a well-corroborated framework for understanding our universe, reports a new preprint study.
This fifth force, mediated by a hypothetical new particle called a symmetron, could guide small “satellite” galaxies into strange orbits around larger galaxies that defy the predictions of the ΛCDM model. In other words, small galaxies captured by the gravitational pull of larger galaxies end up arranged in thin flat planes, or disks, almost like the rings of Saturn, whereas the model suggests they should be distributed in messy orbits all around their host galaxies. Satellites in these synced-up orbits have been seen around our own galaxy, the Milky Way, as well as its closest galactic neighbors, Andromeda and Centaurus A.
Scientists have proposed many possible explanations for this curious gap between theory and observation, known as the “satellite disk problem” or the “planes of satellites problem,” which is one of many small-scale challenges to the ΛCDM model.
Now, a pair of researchers at the University of Nottingham have presented what they believe is “the first potential ‘new physics’ explanation for the observed planes of satellites which does not do away with dark matter,” referring to the unidentified substance that makes up most of the mass in the universe, according to a new study published on the preprint server arXiv.
Aneesh Naik, a research fellow at the University of Nottingham who led the study, said the novel solution emerged from discussions with his colleagues who study particle physics, including co-author and University of Nottingham physicist Clare Burrage, along with his own expertise as an astrophysicist.
“I did my PhD in astronomy and my thesis was looking at galactic dynamics, and thinking about how one can use galactic dynamics to address some fundamental physics problems,” Naik said in a call. “When I got to the end of my PhD, I was at a place where I was thinking a lot about these kinds of small-scale challenges to ΛCDM.”
Indeed, Naik and Burrage note that the ΛCDM model is “a fantastically successful paradigm, accounting for a myriad of independent observations on different scales,” but that “problems begin to emerge when one ‘zooms in’ to small scales” of individual galaxies and their satellites,” according to their new study, which has not yet been peer-reviewed.
Past studies have pointed to the influence of the cosmic web, a giant superstructure that links the universe, as one potential explanation for the tidy distribution of satellite galaxies in thin planes around nearby host galaxies. The orbits may also simply be a feature of our local cosmic neighborhood and not a universal trend, according to other scientists.
Naik and Burrage now propose that particles called symmetrons could generate a special force that creates invisible boundaries in space, known as “domain walls.” Symmetrons are one of many speculative particles that have been proposed to fill in some of the missing links in the standard model, Naik said, to help explain the existence of dark matter and dark energy, which is a weird phenomena that appears to be making the universe expand at an accelerated rate.
“We know that we need new particles because we have dark matter and dark energy and so we suspect that we're going to need to add new particles to our standard model to account for those things,” Naik explained. “That's the context in which people study theories like symmetron theory—it's a new particle candidate for dark energy and/or dark matter.”
Theories suggest that symmetrons have undergone what’s known as symmetry-breaking mechanisms several times over the course of the universe’s 13.8-billion-year lifespan. Essentially, this means that as the universe expands, and accordingly becomes less dense, these particles will pass a threshold density that causes them to flip their lowest-energy state of zero to a random positive or negative value.
“Because the universe isn't even, there will be different low-density regions that are causally disconnected, so this symmetry-breaking actually doesn't happen everywhere in the universe at once,” Naik noted. “Rather, what will happen is that certain places in the universe will first reach that threshold density, and the symmetron will leave its zero solution in that region, and then in a different and causally disconnected region, that symmetry-breaking will happen completely independently.”
“There's a 50/50 chance that two regions will adopt different values,” he continued. “Eventually, you get to a point where these domains have expanded and expanded until what you have is a kind of foam of neighboring domains with positive and negative symmetron solutions. The walls that separate these domains with different solutions are the domain walls.”
Naik and Burrage use simulations to show that interactions along these exotic walls could steer satellite galaxies into the unexpected planes seen around nearby galaxies. The team provides a proof-of-concept for the effect in the study, though it will take much more research to bolster the hypothesis.
“The next stage of investigation is a bit more venturing into the unknown,” Naik said. “These simulations are pretty simplistic. The satellites are just point particles, the domain wall is static, and importantly, there was no friction of any kind.”
“What we can do instead is a proper full cosmological simulation, so really start from cosmological initial conditions and simulate the formation of our Local Group [of galaxies] or have some system that looks very similar to a Local Group,” he continued. “What you can then see is whether it is indeed natural for domain walls to form in our Local Group and whether what you have is the formation of these planes.”
Naik added that it’s “an open question” whether the hypothetical domain walls would even be stable enough to pass through hulking structures such as the Milky Way or Andromeda galaxies, which is yet another enigma that will have to be addressed in future studies.
The answers to these unresolved mysteries are important pieces of the puzzle that is our universe. They may inspire additions or corrections to the ΛCDM model, or perhaps even a wholesale replacement of this stalwart physical framework. After all, the planes of satellites problem is not the only challenge to ΛCDM, and more may well arise in the future.
To that end, the new study may represent just one step toward a better understanding of the standard model, but Naik concluded that there is also an “interesting wider context” that demands further exploration of “all the places where ΛCDM seems to break down on galaxy scales.”